(1) Field of the Invention
This invention relates to a pointed heat-generating device for molds of injection molding machines which enables an accurate injection molding operation.
(2) Description of the Prior Art
In FIGS. 1 to 3 there is shown a conventional pointed heat generating device of this kind. It is disclosed in U.S. Pat. No. 4,516,927 and European Pat. No. 0 043 234 B1, both of which correspond to Japanese Unexamined Patent Publication No. 57-11017 in the name of the present applicant.
Referring to FIG. 1, numeral 1 is a projectile heat-generating body 1 which comprises a cylindrical metal casing 2 and a pointed conical heat-generating means 3 fixed therewith.
The pointed conical heat-generating means 3 is provided, on its axis, with a passage 4. Penetrated wholly into the passage 4 is a rod-type heating wire 5 until its end reaches a pointed end of the heat-generating means 3. The rod-type heating wire 5 which is a nichrome wire is inserted into an insulating tube 5a.
Associated with the rod-type heating wire 5 is an alloy portion X of wear resistance which is nearly equivalent to a material of the heat-generating means 3.
How to make the alloy portion X will now be described in connection with FIGS. 2 and 3.
First of all, the passage 4 is bored in a cylindrical non-processed material 3', and a nichrome wire inserted into the insulating tube 5a is inserted as the heat-generating wire 5 into the passage 4. An end of the cylindrical non-processed material 3' and an end of the heat-generating means 3 are fused locally and united to each other in such an inert gas as argon and converted into an integral part, i.e. the alloy portion X by means of arc discharge.
Thus, the alloy portion X has the nearly same wear resistant property as the heat-generating means 3. By cutting and processing the cylindrical non-processed material 3', there is formed the pointed heat-generating means 3 having the alloy portion X as a peak.
The alloy portion X is connected to an end of the heating wire 5 and earthed to the metal casing 2 by way of the heat-generating means. Another end of the heating wire 5 is connected to a back lead wire 6 by passing through the passage 4.
Numeral 7 is a heater which is coiled along an internal circumpherence of the metal casing 2. The coiled heater 7 is to heat and fuse the thermoplastic synthetic resin which flows and retains along the external circumpherence of the metal casing 2. Numeral 8 is a lead wire connected to the heating wire 5.
Since the heating wire 5 is incorporated longitudinally within the heat-generating body 1 and not exposed outwardly, the pointed heat-generating means 3 can maintain a high wear resistance. However, the disadvantage of the heating wire 5 is that it is simply rod-type, so that the speed of heat conductivity is delayed. In order to obtain a high temperature heat rapidly and effectively, such conventional heating wire has needed electric power of more than 120 W, 8 volts and 15 amperes. Accordingly, a large-sized transformer having high capacity is absolutely required even when producing small-sized molding articles. Thus, it raises the production cost.